The Funny Current

Put two fingers to your throat and you find a clock that nobody is winding. The beat arrives, and arrives again, and the strange thing is that nothing outside is sending it. There is no metronome in the chest, no signal coming down from the brain to say now, and now, and now. A small knot of tissue at the top of the right atrium, the sinoatrial node, is making the beat out of itself. Cut every nerve to the heart and it keeps time anyway. It is one of the few clocks in the body that needs no input to run, and understanding how it manages that turns out to be a lesson in the difference between a thing that starts and a thing that waits.

Most cells in the body sit still until they are told otherwise. A nerve cell, a muscle cell, holds a stable resting voltage — negative inside, poised, patient — and does nothing at all until a signal arrives to tip it over threshold. It is a thing that waits. The pacemaker cell cannot do this, and that incapacity is the whole secret. It has no stable rest. The moment it finishes one beat and settles toward its low voltage, it begins, immediately and on its own, to drift back upward — a slow leak of charge across the membrane during the pause between beats that electrophysiologists call the diastolic depolarization. The cell creeps toward threshold all by itself, reaches it, fires, falls back, and starts creeping again. For the pacemaker, drifting upward is its rest. It keeps time not because something pushes it but because it cannot hold still, and the rate at which it can't hold still is the rate of your pulse.

The engine of that upward creep was, for a quarter of a century, hiding inside a mistake. When researchers first found a current involved in the rhythm of cardiac fibers, they misread it — they thought they were watching a potassium current slowly switch off, and they named it accordingly. It was Dario DiFrancesco who, working through the late 1970s and into a careful reinterpretation in 1981, showed that the apparent decay of an outward current was actually an inward current quietly switching on. The thing had been mislabeled because it behaves backward. Most ion channels in the body open when a cell depolarizes, when it grows less negative; this one does the opposite — it opens when the cell hyperpolarizes, when it sinks toward its most negative point, exactly the moment a tired cell should be going quiet. Because that behavior was so contrary to expectation, the people who studied it gave it a name that stuck: the funny current, I-f. It is funny in the way an inverted answer is funny. The current that restarts the heart turns on precisely when everything else is shutting down.

That single self-starting node is not, it turns out, the only cell in the heart that can keep time. Run down from it and you find others — the atrioventricular node, the Purkinje fibers threaded through the ventricles — and every one of them has the same restless inability to sit still, the same upward drift. They are all latent clocks. The reason you have one heartbeat and not three is simply speed: the sinoatrial node drifts to threshold fastest, around a hundred times a minute left to itself, and each time it fires it discharges everything downstream before those slower clocks can reach their own threshold. It resets them, every beat, just shy of their moment. The atrioventricular node, left alone, would settle near forty or fifty; the ventricular fibers, slower still, near twenty or thirty. They keep their own counsel and stay silent, overruled once a second, for years on end. And if the node at the top ever falls quiet — if the dominant clock stops — one of the silent ones, no longer interrupted, simply rises to its own threshold and takes over. The cardiologists call it an escape rhythm, and the word is exact: a slower beat that was there the whole time, escaping at last into the gap left by the faster one's absence.

The dominant clock does not merely outrun the others; it actively holds them down. Drive a pacemaker faster than its own natural rate and you load its cells with sodium, which wakes a pump in the membrane that trades sodium back out for potassium and, in doing so, pulls the voltage more negative — pushes the cell further from threshold than it would otherwise sit. So the slower clocks are not just outpaced; they are suppressed, dragged down by the very pace imposed on them, and when the overdrive finally stops the pump keeps running a moment longer, so there is a pause — a held breath — before the freed clock can climb back to threshold and beat. The hierarchy is maintained, in other words, by the leader continuously pressing the followers below the line at which they could lead.

We have built our own version of this knot of tissue, and the way we built it reveals which kind of clock we find easy to imagine. The first crude attempts simply fired, blind. Albert Hyman coined the term artificial pacemaker for the hand-cranked apparatus he built in 1932, a thing that delivered its pulse on a fixed schedule and paid no attention to the heart it was prodding. The first pacemaker ever implanted in a person — given to a Swede named Arne Larsson in 1958 — was just as blind, and failed within hours; Larsson went through some two dozen of them over the next four decades and outlived both the surgeon who implanted the first and the engineer who built it. But the device almost everyone now carries is cleverer and stranger than that, and it is built on the opposite principle from the cell it replaces. A modern demand pacemaker, in its commonest mode, does not generate a beat. It listens for one. It runs a timer, and every time it senses the heart's own beat it hears the beat and resets, firing nothing. Only when the expected beat fails to arrive — only on absence — does it finally discharge and fill the gap. It is defined entirely by what is missing. The biological pacemaker is the thing that cannot stop starting; the machine we made to stand in for it is the thing that does nothing until something is absent. The body's clock generates; ours, in demand mode, waits.

There is an irony folded into that. We picture the living thing as the one that responds and the machine as the one that runs on its own, and here it is exactly reversed: the flesh is the self-starter, and the device we built to stand in for it is the reactor. One generates the beat; the other waits for the beat's absence and fills it. A demand pacemaker can be reliable, can be lifesaving, can fill ten thousand gaps without complaint — but it has no tempo of its own; it only ever answers. The knot of tissue at the top of the atrium was never asked to beat; since before you were born, it has simply never held still.